The most common and most likely answer to this crossword clue is the 11 letter word EQUILATERAL
I've seen this crossword clue in The Guardian
E
Q
U
I
L
A
T
E
R
A
L
Buddy explains!

The straight part
We know that "square, possibly" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case square, possibly = equilateral.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.
What does EQUILATERAL mean?
EQUILATERAL noun- A side exactly corresponding, or equal, to others.
- A figure having all its sides equal.
- (of a polygon) Having all the sides equal.
- (of a polyhedron) Having all the faces equal.
- Having the two sides equal, as a surface divisible by a longitudinal median line into two halves of the same form.
- Having all the convolutions of the shell in one plane, chiefly of foraminifers.